Speed limit

Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed, expressed as kilometres per hour (km/h) or miles per hour (mph) or both. Speed limits are commonly set by the legislative bodies of national or provincial governments and enforced by national or regional police and judicial authorities. Speed limits may also be variable, or in some places nonexistent, such as on most of the Autobahnen in Germany.

The first numeric speed limit for mechanically propelled road vehicles was the 10 mph (16 km/h) limit introduced in the United Kingdom in 1861.

Speed limits are frequently introduced for health reasons with a goal to improve road traffic safety and reduce the number of casualties from traffic collisions. There is a relationship between speed and health impacts, whereby it has been posited that a 5% increase in speed is linked to a 10% increase in injury, and a 20% increase in deaths. Speed limits tend to be stricter in areas with more pedestrians, in particular those frequented by children. The World Health Organization (WHO) identified speed control as one of a number of steps that can be taken to reduce road casualties. As of 2021, the WHO estimates that approximately 1.3 million people die of road traffic crashes each year.

Authorities may also set speed limits to reduce the environmental impact of road traffic (vehicle noise, vibration, emissions) or to enhance the safety of pedestrians, cyclists, and other road-users. For example, a draft proposal from Germany's National Platform on the Future of Mobility task force recommended a blanket 130 km/h (81 mph) speed limit across the Autobahnen to curb fuel consumption and carbon emissions. Some cities have reduced limits to as little as 30 km/h (19 mph) for both safety and efficiency reasons. However, some research indicates that changes in the speed limit may not always alter average vehicle speed. Lower speed limits could reduce the use of over-engineered vehicles.

In Western cultures, speed limits predate the use of motorized vehicles. In 1652, the American colony of New Amsterdam passed a law stating, "No wagons, carts or sleighs shall be run, rode or driven at a gallop". The punishment for breaking the law was "two pounds Flemish", the equivalent of US$50 in 2019. The 1832 Stage Carriage Act introduced the offense of endangering the safety of a passenger or person by "furious driving" in the United Kingdom (UK). In 1872, then-President of the United States Ulysses S. Grant was arrested for speeding in his horse-drawn carriage in Washington, D.C.

A series of Locomotive Acts (in 1861, 1865 and 1878) created the first numeric speed limits for mechanically propelled vehicles in the UK; the 1861 Act introduced a UK speed limit of 10 mph (16 km/h) on open roads in town, which was reduced to 2 mph (3 km/h) in towns and 4 mph (6 km/h) in rural areas by the 1865 "Red Flag Act". The Locomotives on Highways Act 1896, which raised the speed limit to 14 mph (23 km/h) is celebrated by the annual London to Brighton Veteran Car Run.

On 28 January 1896, the first person to be convicted of speeding is believed to be Walter Arnold of East Peckham, Kent, UK, who was fined 1 shilling plus costs for speeding at 8 mph (13 km/h).

In 1901, Connecticut was the first state in the United States to impose a numerical speed limit for motor vehicles, setting the maximum legal speed to 12 mph (19 km/h) in cities and 15 mph (24 km/h) on rural roads. Speed limits then propagated across the United States; by 1930 all but 12 states had established numerical limits.